1. Field of the Invention
The present invention relates to an optoelectronic device for reading data contained on a magnetic medium. It especially applies to magneto-optical memories chiefly used in data processing system.
2. Description of the Prior Art
It is known that data processing systems basically contain a central processing unit and a plurality of input/output devices, also called "peripheral units". These systems are used to enter information into the central processing unit (CPU) where arithmetic logic units process the information. After processing is completed, the CPU sends the information to the output devices. The result of the operations performed by the CPU is then used and analyzed immediately by the user of the information-processing system or else is stored for variable periods of time into memories, most often called "mass storage".
The two most frequently used forms of mass storage are magnetic disc memories and tape drives.
In magnetic disc memories, the information is contained on magnetic discs within circular, concentric recording tracks whose radial width is on the order of several tens of microns. These tracks usually cover the major portion of the two surfaces of the disc.
In magnetic tape drives, the information is stored on magnetic tape within tracks which are parallel to the length of the tape.
Generally, a series of magnetic bits of information recorded on a track of a magnetic disc or tape appears in the form of a succession of small magnetic domains, called "elementary domains", which are distributed over the entire length of the track and have magnetic inductions of equal magnitude and opposite direction.
The means which allow data to be recorded on discs or tapes, or to be read, or both of these functions to be performed, are called transduction devices or, more simply, transducers. Generally, a given storage device (disc or tape) has one or more associated transducers, with the storage device moving past such transducer(s).
There are two major types of magnetic disc memories. These are disc-type memories and magneto-optical memories.
Disc-type memories will be designated as "conventional disc memories". The reading and writing functions are performed by magnetic transducers generally consisting of a magnetic circuit surrounded by windings and which includes a magnetic core. Variations in the induction within the core of such a transducer allow the reading and/or recording of the data contained in the storage device associated with this transducer.
The longitudinal density (or line density) is defined as the number of bits of information per unit of length measured in relation to the circumference of a track, in the case of a magnetic disc, or in relation to the length of the tape, in the case of a magnetic tape.
The radial density (in the case of a magnetic disc) is the number of tracks per unit of length measured in relation to the diameter of the disc.
The term "bit" is used to designate both the binary information unit which is equal to 1 or to 0, and any physical representation of this information which, for example, could be an elementary magnetic domain contained on a track of magnetic disc or also an analog or logic electrical signal.
Current conventional disc memories achieve radial densities on the order of 350 to 450 tracks per centimeter (i.e., 850 to 1,000 tracks per inch expressed as 850 to 1,000 TPI), whereas longitudinal densities are on the order of 2,000 bits per centimeter, corresponding to about 5,000 bits per inch,(expressed as 5,000 bpi). Magneto-optical memories have the writing function performed for the most part by magnetic means, whereas the reading function is performed by an optoelectronic apparatus which includes a set of optical devices and photoelectric transducers which transform the light they receive into an electrical signal. In other words, magneto- optical memories are memories in which data is stored on magnetic discs and read by optoelectronic devices. The radial and longitudinal densities obtained are on the order of 25,000 TPI and 25,000 bpi, respectively. Thus, the size of the elementary magnetic domains is on the order of 1 to 2 microns, and the width of the tracks is of the same magnitude.
Magneto-optical memories use two major modes of writing data, namely, a "thermo-magnetic" writing mode and a purely magnetic writing mode using magnetic transducers which may be of the type described above. The principle of the thermomagnetic mode of writing is to use the termal effect of the impact of an electromagnetic laser beam on the magnetic material forming the recording surface of the magnetic discs. The laser beam is made of coherent mono-chromatic electromagnetic radiation.
Electromagnetic radiation (which can be called light in a general sense) is linearly polarized in the plane when the electric-field vector (and, subsequently, the magnetic induction vector) continuously maintains the same direction in the plane which is perpendicular to the direction of propagation of the wave. This is true for any position of this plane in space, at any instant of observation. The polarization plane is defined as the plane which contains the direction of propagation of light and the electric-field vector.
Electromagnetic radiation is said to be coherent when, for example, in the case of linearly polarized light in the plane, the phase of the electric field and, thereby, the phase of the magnetic induction field, are identical at all points of another identical plane which is perpendicular to the direction of propagation.
To write a bit on a magnetic disc, using the thermomagnetic mode of writing, requires that a focused laser beam be generated, the size of that beam being of the same magnitude as the bit to be written, i.e., one or two microns. It is assumed that initially the magnetization of the magnetic film constituting the magnetic discs is uniform, i.e., the magnetic induction at every point has the same direction and the same magnitude. At the spot where the laser beam strikes the surface of the magnetic disc, this surface is heated to the point where the temperature of the magnetic film at that spot becomes much higher than the Curie T.sub.c temperature of the magnetic material, which is the temperature at which the magnetic material begins to lose its magnetization. If that spot is then subjected to a magnetic field in the direction opposite to that of the magnetization in the magnetic film, the magnetic material, upon cooling (the laser beam having then been turned off), will then become magnetized in the direction opposite to that of the uniform magnetization of the rest of the magnetic film on the magnetic disc.
The data reading mode (in magneto-optical memories) is based on the principle of interaction between polarized light and the magnetic state of the film which constitutes the magnetic disc, and that interaction produces a rotation of the electric-field vector in the plane which is perpendicular to the direction of propagation (and also of the magnetic field of the electromagnetic radiation which constitutes polarized light). For this purpose, a beam of linearly polarized light, preferably monochromatic, is emitted (such polarized light may be, for instance, a laser beam), and this beam is focussed so that its size is on the order of the magnetic domain constituting data contained on the magnetic disc. If the magnetic medium is assumed to be such that its magnetization is perpendicular to the surface of the film, which is called "perpendicular magnetization" (a longitudinally magnetized magnetic medium could also be used, i.e., where the magnetization would be parallel to the film itself), it is observed that after the incident beam has been reflected by this film, the electric-rield vector of the polarized light is rotated in the plane which is perpendicular to the direction of propagation of the light, which by convention is said to be equal to angle (-o) when the beam of light strikes a domain having negative magnetization and equal to (+o) when the beam of light strikes a domain having positive magnetization. The physical phenomenon which has just been described (the interaction between light and the magnetic state of the material producing a rotation of the field vector and of the electric-rield vector) is called the "Kerr effect". (The Kerr effect is said to be polar f the magnetization of the magnetic field is perpendicular, and to be longitudinal if the magnetization of the film is ong:tudinal).
It can be seen that in order to determine the value of a bit, all that is needed is to detect the rotation of the electric field vector. This is accomplished by means of a device called an analyzer, which consists of a crystal favoring one direction of propagation of light, which is positioned in such way that this direction is 90.degree. away from the direction of the electric-field vector of reflected light if that light has been reflected by a magnetic domain having negative magnetization. Under these conditions, a light of zero intensity is collected at the output of the analyzer. On the other hand, when that light is reflected on a magnetic domain having positive magnetization, a light of positive intensity appears at the output of the analyzer. Put differently, the domains having negative magnetization will appear as black spots on a screen placed at the output of the analyzer, while domains having positive magnetization will appear as light spots.
It is obvious that if photoelectric transducers (e.g., silicon photodiodes) are placed at the output of the analyzer, the signal produced by the photoelectric transducer will have zero voltage (or zero current) in the presence of a domain having negative magnetization, and a non-zero voltage in the presence of a domain having positive magnetization.
Opto-electronic magnetic data reading devices implementing the principles stated above are now well known. One of these devices is described, for example, in an article by NOBUTAKE IMAMURA and CHUICHI OTA entitled, "Experimental Study on Magneto-optical Disc Exerciser with the Laser Diode and Amorphous Magnetic Thin Films", published in the Japanese Journal of Applied Physics, volume 19, No. 12, Dec. 1980, pp. L 731-34.
This optoelectronic device includes a polarized laser beam emitter; a separator element separating an incident beam and a reflected beam; a device for focussing the laser beam on the surface of the magnetic disc containing the data to be read; a light analyzer; and a photoelectric transducer.
The polarized laser beam is directed by the laser beam generator through the separator element and the focussing device onto the surface of the disc in such a way that it is perpendicular to the disc and that it is of the same magnitude as the magnetic domains. After reflection by the surface of the disc, the reflected laser beam is transmitted to the light analyzer by means of the separator element. It is then collected at the output of the analyzer by the photoelectric transducer.
The device for focussing the beam onto the surface of the disc is controlled in such fashion that, independently of any oscillations of the surface of the disc (the surface of a disc is never completely flat and the disc always rotates slightly out of true), the laser beam is always precisely focussed onto the surface of the disc.
In addition, as soon as the track to be read has been selected and the reading device adjusted to face the selected track, the laser beam should remain perfectly centered on the magnetic domain of the track, which contains the data to be read. For this purpose, the laser beam is divided into three viewing beams, with the central beam being used to read the data, while the two lateral viewing beams on either side of the track are used to center the device in relation to the track to be read.
While the reading device briefly described above allows magnetic discs containing a large amount of data to be read (it is recalled that the densities of such discs which are used in magneto-optical memories contain between 100 and 200 times more data per surface area than magnetic discs used in conventional disc memories), it has the disadvantage of being bulky, contains highly accurate--thus costly--focussing and centering devices, and has a reading output signal such that the signal-to-noise ratio at the output of the photoelectric transducer is limited. This is the reason why existing magneto-optical memories are relatively seldom used as compared to conventional magnetic disc memories.